3rd generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future.
An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference with a low complexity is taken into consideration as one of next generation (after the 3rd generation) systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased. Therefore, inter-symbol interference is minimized.
In a system using the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. The OFDMA is adopted for uplink and downlink transmission in institute of electrical and electronics engineers (IEEE) 802.16 and 3rd generation partnership project 2 (3GPP2) ultra mobile broadband (UMB) and for downlink transmission in 3GPP long term evolution (LTE).
FIG. 1 is a block diagram showing a structure of a typical OFDMA transmitter. Referring to FIG. 1, the OFDMA transmitter includes an encoder, a modulator, a digital/analog (D/A) converter, a frequency region mapper, an inverse fast Fourier transform (IFFT) processor, an analog/digital (A/D) converter, a cyclic prefix (CP) inserter, and a radio frequency (RF) transmitter. In the OFDMA transmitter, the encoder first encodes data to be transmitted. Then, the modulator receives the encoded data and converts the received into a serial symbol in a frequency region. The D/A converter converts serial data symbols into parallel data symbols. The frequency region mapper allocates the converted parallel data symbols to subcarriers having orthogonality. The IFFT processor converts the parallel data symbol, which has been converted in the frequency region, into a time region signal. The CP inserter inserts a CP to data output from the IFFT processor. The RF transmitter transmits the CP-inserted data.
One of main problems of the OFDMA system is that power efficiency of signals can be low due to a significantly large peak-to-average power ratio (PAPR). The PAPR problem occurs when a peak amplitude of a transmit (Tx) signal is significantly larger than an average amplitude. Further, the PAPR problem is caused by a fact that an OFDM symbol is an overlap of N sinusoidal signals on different subcarriers. A high PAPR is not a significant problem in a base station (BS) in which power is not limited relatively. However, the high PAPR is problematic in a user equipment (UE) which has limited maximum power and of which a power amplifier is limited in efficiency. Therefore, not to mention that the OFDMA is adopted as a multiplexing scheme for downlink, when the OFDMA is adopted as a multiplexing scheme for uplink, power consumption of the UE increases, resulting in decrease in the coverage of the BS.
Single carrier-frequency division multiple access (SC-FDMA) is proposed to decrease the PAPR. The SC-FDMA is single carrier-frequency division equalization (SC-FDE) combined with frequency division multiple access (FDMA). The SC-FDMA is similar to the OFDMA in that data is modulated and demodulated in a time region and a frequency region by using discrete Fourier transform (DFT). However, the SC-FDMA is advantageous to decrease Tx power since a Tx signal has a low PAPR. In particular, regarding battery usage, the SC-FDMA is advantageous in case of uplink transmission where communication is achieved from a UE sensitive to Tx power to the BS. Due to these advantages, the SC-FDMA is adopted as a multiplexing scheme for uplink in the 3GPP LTE, i.e., a 4th generation mobile communication technique.
FIG. 2 is a block diagram showing a structure of a typical SC-FDMA transmitter. Referring to FIG. 2, the SC-FDMA transmitter includes an encoder, a modulator, a D/A converter, a discrete Fourier transform (DFT) processor, a frequency region mapper, an IFFT processor, an A/D converter, a CP inserter, and an RF transmitter.
The structure of the SC-FDMA transmitter is very similar to that of the OFDMA transmitter except that the SC-FDMA transmitter further includes the DFT processor and except that the modulator and D/A converter process time region data whereas the OFDMA transmitter processes frequency region data. More specifically, in the SC-FDMA transmitter, the encoder first encodes data to be transmitted. Then, the modulator receives the encoded data and converts the received into a serial symbol in a time region. The D/A converter converts a time region serial data symbol into a parallel symbol. An output of the D/A converter is input to the DFT processor, is converted into a frequency region signal, and is then input to the frequency region mapper. The frequency region mapper allocates the converted parallel data symbol to a subcarrier. The IFFT processor converts the parallel data symbol, which has been converted in the frequency region, into a time region signal. The CP inserter inserts a CP to data output from the IFFT processor. The RF transmitter transmits the CP-inserted data.
As such, the SC-FDMA transmitter processes data in the time region and arranges data also in the time region when transmission is performed. Therefore, the PAPR is decreased and thus efficiency of a power amplifier can be increased. Accordingly, it is effective to use the SC-FDMA transmitter when the UE is sensitive to power consumption. In particular, the use of the SC-FDMA transmitter is effective for UEs located in a cell boundary. However, the SC-FDMA has a disadvantage in that transmission efficiency in a multi-path fading channel is generally low in comparison with the OFDMA. In addition, the SC-FDMA has another disadvantage in that high transmission efficiency is difficult to be achieved since its performance is poor in a high-order modulation scheme such as quadrature amplitude modulation (QAM) in comparison with the OFDMA and since it is unfavorable to use a maximum likelihood (ML) multiple input multiple output (MIMO) detector or a low complex transform receiver in a multiple antenna system (i.e., MIMO system).